Asking for review / validation : making a very simple AC transformer box

[Ian.M]

Assuming the interwinding capacitance is between the outer layer of one secondary and the inner of the other, they are effectively in series with the capacitance between them.  I estimate the capacitance is around 200pF, reasonable for the transformer size and construction.

A simple calculation from Zc=1/(2𝜋C) gives 187pF, but that ignores the voltage drop across the end layer of each winding.  Depending on the number of turns per layer and total turns, the actual capacitance could be a factor of two greater.

[dobsonr741]

Another measurement worth making: AC voltage from the red or black to protective ground. You will be surprised!

[Ian.M]

Yes, and again, see what difference the 10K makes, and if its different for the two secondaries when they are independent.

You've still got enough room on that panel above the power switch to add a binding post for Mains PE ground, which is useful to have if the circuit you are working with needs a ground reference, and you said earlier you had a spare green binding post . . .

[gwelavo]

Interesting. I will do the measurements tonight. Just to be sure I understand correctly, in order to reproduce figure 2L.10 of the book (see attachments), I just need to make sure the "output" is plugged to earth (either the one of the function generator or the earth from my power supply, they are the same).

In case I have a circuit that depends on earth, I will then need to connect either the black or the red jacks to earth in order to avoid the difference of potential.

[EPAIII]

If you are going to go that route, why not get one with the fuse holder also built in. One part gets all of it: cable entry with disconnect ability, filter, and accessible fuse. And you only need to make one mounting hole in the box.

These days it is common to include a RFI filter on the mains input of power supplies.  You may not need it..  You can purchase them cheaply.  They come with the proper X and Y capacitors. I prefer the ones with the three prong plug.

This is what I am talking about. Not this one but a new one. Or possibly one from a discarded electronic component you have.  There are better ones  that are bigger and do not have the three pronged plug.


https://i.ebayimg.com/images/g/nLAAAOSw~KpgCgzO/s-l1600.jpg